Why can't everyone just have their own beliefs?

Everyone does have their own beliefs, but all beliefs are not equally valid nor do they equally reflect reality. An incorrect belief may occasionally be harmless, but it can also directly result in negative consequences (unnecessary guilt, bigotry, racism, oppression, strife, silencing of dissent, the death of the innocent, etc.)

Beliefs direct and guide our actions, and incorrect beliefs can lead to actions that increase harm, both for the individuals holding a particular belief and those around them. A perfect example of this is the preventable deaths of children at the hands of those who wait for a god to heal instead of taking their children to a doctor. If someone dies because their faith kept them from seeking medical treatment, it is tragic. It is far more so when an innocent child is lost do to their parent’s ignorance and their reliance on a belief that is not supported by logic, evidence or reason.

Another example of beliefs harming others is the oppression and hatred directed at homosexuals, atheists and others.

Teaching a child that they are a sinner, a worm that deserves eternal damnation, can do incredible harm to a child and can lead to unhealthy levels of guilt and repression. The belief that condoms are evil has helped increase the levels of STD’s and unwanted pregnancy all across the world. This is directly evidenced by the prevalence of HIV in Africa.

A belief or opinion that is based on lies, falsehood or misunderstanding will have consequences. It is better that our beliefs and opinions reflect the world around us, and it is often necessary to change our beliefs as new evidence becomes available.